Membrane distillation (MD) is a thermally driven separation process used in water treatment. Main applications of MD include notably water desalination, reverse osmosis process, and removal of organic matter in drinking water production. In a typical MD process, a temperature gradient is first applied between two opposing sides of a porous hydrophobic membrane, thus forming a “hot side” and a “cooling side” of the membrane. Subsequently, a liquid feed from sea water or other non-potable water sources is heated at the hot side of the membrane, and the porous membrane functions as a physical support and a liquid barrier to keep the hot fluids from entering the cooling side while allowing the vapour generated from the hot fluids to migrate to the cooling side. The cooling side of the membrane can contain either a liquid or a gas to collect and condense the migrated vapour molecules. In a commonly used MD configuration, Direct Contact Membrane Distillation (DCMD), a hot saline feed is separated from a cold liquid flow at the cooling side by the porous membrane.
In order for a vapour-permeable membrane to properly function as a robust liquid barrier in the MD application, it is essential to have a high surface hydrophobicity at least on the fluid-contacting side of the membrane. For this reason, the hydrophobic porous membranes used for microfiltration or ultrafiltration process have been conveniently applied in the MD process.
A variety of hydrophobic materials have been used to construct the porous membrane for MD in academic research and some commercial equipments, including Polypropylene (PP), Poly(vinylidene fluoride) (PVDF) and Polytetrafluoroethylene (PTFE), either in flat sheet or hollow fiber configuration. However, the composition of these existing hydrophobic membranes is not optimized for the MD application. Particularly, on one hand, a MD membrane formed of the aforementioned hydrophobic materials must be made relatively thick: not only to provide the necessary mechanical support but also to function as an effective heat conduction barrier in the thermally driven MD process. On the other hand, in order to maximize the water vapour flux across the MD membrane (i.e. the measure of MD process productivity), it is highly desirable to minimize the thickness of the MD membrane.
Recently, as an attempt to solve the aforementioned dilemma, several types of thin film composite (TFC) membranes were tested in the aforementioned DCMD configuration, each made of two layers: one thin hydrophobic layer attached to one thicker hydrophilic sub-layer as described in QTAISHAT, M., et al. Guidelines for preparation of higher flux hydrophobic/hydrophilic composite membranes for membrane distillation. Journal of Membrane Science. 2009, vol. 329, p. 193-200. and BONYADI, S., et al. Flux enhancement in membrane distillation by fabrication of dual layer hydrophilic-hydrophobic hollow fiber membranes. Journal of Membrane Science. 2007, vol. 306, p. 134-146. In use, the thin hydrophobic layer in said TFC membrane directly contacts the feed fluid in DCMD and withholds liquid from the pores, while the hydrophilic sub-layer provides the necessary physical support and functions as an effective thermal barrier between the two membrane sides. Both the reduced thickness of the hydrophobic layer and the increased pore hydrophilicity in such a TFC membrane desirably reduce the vapour flux resistance and mitigate temperature polarization effects.
Additionally, the use of a fluoropolymer coating to increase the surface hydrophobicity of a MD membranes has been disclosed in US 2006/0076294 A (KAMALESH K. SIRKAR, BAOAN LI,) Apr. 13, 2006. In this application, the hydrophobic coating is applied to a porous membrane using plasma polymerization or solution deposition methods to further decrease membrane pores wetting. However, no details of the coating preparation were reported in this application. Plasma polymerization is generally recognized as a complex method to apply in membrane manufacturing and thus has limited industrial application. Solution coating can be simpler in practice, but does not generate strong chemical bonding between the fluoropolymer coating and the supporting membrane. As a mere physical adhesion of fluoropolymer coating with other surfaces is known to be extremely poor, the durability of a fluoropolymer coating made by solution coating according to US 2006/0076294 is expected to be undesirably low.
Thus, there is a need in the art for hydrophobic/hydrophilic composite membrane which can be easily manufactured and still provides outstanding thermal stability and physical durability properties.